Geraldine Somerville spent a lot of time during the shoot for the ITV series Titanic being extremely damp. "It was really challenging," she says, recalling the filming in Budapest, where the largest indoor water tank in Europe (900 sq m) had been specially constructed. "It was boiling, boiling hot weather and we were trying to pretend we were really, really cold… When we were in the water tanks, you saw the steam rising around us. We stank. There was something in the lifejackets which, when we sweated, it was horrible – like hamster cages that haven't been cleaned out."

Such indignities aside, Somerville, 44, enjoyed her time on set playing the snobbish Countess of Manton in Julian Fellowes's dramatisation to mark the 100th anniversary of the liner's sinking. It is his first project for ITV since Downton Abbey and the all-star cast includes Celia Imrie, Toby Jones and Linus Roache. Is Somerville a Downton fan?

"I am. The thing about Julian is that he tells a good story and, at the same time, he almost educates his audience about the mores of certain types of people. You get that in both Downton Abbey and Titanic: the codes of behaviour between certain classes."

Somerville herself comes from somewhat aristocratic stock – she is the daughter of a baronet and is married to the stockbroker William Osborne-Young, with whom she has three children.

Somerville trained at Guildhall School of Music and Drama. Bit parts in Casualty and Poirot followed, but it was her brilliant performance as DS Jane Penhaligon alongside Robbie Coltrane's criminal psychologist in Cracker in the mid-1990s that brought her critical acclaim.

Since then, she seems never to have been out of work – all three children were fitted in around her role as Harry Potter's mother in the film adaptations. "For those 10 years, I had either just had a baby or was breastfeeding one," she laughs. "It was such a womby time."

She remains "terribly fond" of her onscreen son, Daniel Radcliffe. "I feel maternal towards him. When something happens or I read something bad about him, I get this pull on the heartstrings. It's bizarre."

Is there a distinct point when, as a female actor, one starts getting offered "mother" roles?

"For me, it's evolved over time, but I found it quite strange, obviously, in Titanic that my daughter is a twentysomething. It's the way of the world that we're all fascinated by youth. I think I'm at a tricky stage… it's quite a hard age and quite difficult to find good roles. It's odd, because I am my age, my friends are my age and we all have youngish children, and we don't get represented that much in the media."